Because so far NATO’s reaction to Putin’s aggression has been to send a handful of forces to the Baltics to demonstrate ‘resolve,’ which has only convinced Putin that the alliance is either unable or unwilling to fight. So we had better change his calculus pretty soon, and contest Putin’s stated doctrine that he is willing to intervene militarily in other countries to ‘protect’ Russia-speaking people. For God’s sake, the last time we heard that was just before Hitler invaded the Sudentenland.